XftFontMatch does display-specific font configuration (commit 528241a).
Nice. Unfortunately, when we switched from FcFontMatch, we also stopped
storing the post-Fc{Config,Default}Substitute FcPattern for future
lookups. The result is that if a glyph isn't found in the primary font,
secondary font lookups use the original FcPattern, not the configured
one. If you have custom fontconfig rules (like me), this can be
disappointing.
I basically just copied the guts out of XftFontMatch[1] and saved
the intermediate configured FcPattern. Could be related to the bug that
inspired commit 4242027.
[1]: https://cgit.freedesktop.org/xorg/lib/libXft/tree/src/xftfont.c